Conventionally, there are various types of power sources for various devices that require a large amount of power. A battery pack that includes a plurality of electric cells and a packaging case accommodating the plurality of electric cells is known as an example of the power sources.
In recent years, hybrid electric vehicles that emit less CO2 and electric vehicles that do not emit CO2 (hereinafter, the vehicles will be collectively referred to as “electrically powered vehicles”) are widely used from the viewpoint of environmental protection. The battery packs are adopted as driving sources of the electrically powered vehicles and as power sources for supplying power to control systems.
This type of battery pack adopts a rechargeable secondary cell as an electric cell. However, the electric cell (secondary cell) tends to generate heat due to charge and discharge, and an excessive temperature rise of the electric cell causes a reduction in performance of the electric cell and a shortened life span.
Consequently, a battery pack has been proposed in which a plurality of electric cells are aligned and accommodated in a packaging case, and first and second passages are arranged with the plurality of electric cells interposed therebetween.
In the battery pack, the plurality of electric cells are arranged so that ventilation spaces through which gas flows are formed between adjacent electric cells. Therefore, if gas, such as open air, is supplied to the first passage, the gas flows from the first passage to the second passage through the ventilation spaces (cf., JP-A-2000-243461, for example).
As a result, the gas flowing though the ventilation spaces cools the electric cells on both sides of the ventilation spaces in the battery pack, preventing an excessive temperature rise of the electric cells. It is therefore supposed that the life span of the electric cells is not reduced, and the electric cells can provide necessary performance.
However, the gas is supplied to the first passage from a predetermined location of the first passage in the battery pack, and therefore the electric cells in a downstream area of the first passage may not be sufficiently cooled.
Specifically, if the number of electric cells arranged in the packaging case is larger, the length of the first passage is longer in accordance with the number of electric cells. Therefore, the pressure loss of the first passage cumulatively increases from the upstream to the downstream. More specifically, a large amount of gas flows into the ventilation spaces in an upstream area with a small pressure loss in the first passage, while the gas is not sufficiently supplied to the ventilation spaces in the downstream area with a large pressure loss. Therefore, the electric cells in the downstream area of the first passage may not be sufficiently cooled in the battery pack.
Therefore, not only the possibility of a breakdown of the electric cells in the downstream area of the first passage increases in the battery pack, but there are also variations in temperature between the electric cells in the upstream area of the first passage and the electric cells in the downstream area of the first passage. There is a problem that the performance of all electric cells cannot be sufficiently achieved.